Method and apparatus for making cylindrical printed circuits



Jan. 10, 1961 M. A. WETMORE ETAL 2,967,766

METHOD AND APPARATUS FOR MAKING CYLINDRICAL PRINTED` CIRCUITS Filed0G12. 22, 1957 Flai FIGO 2 INVENTORS ZcZz/zfz X Weimar@ (QW i www UnitedStates Patent O METHOD AND APPARATUS FOR MAKING 'CYLINDRICAL PRINTEDCIRCUITS Melvin A. Wetmore, Norwalk, and William H. Howell,

Fairfield, Conn., assignors to Aladdin Industries, Incorporated,Nashville, Tenn., a corporation of Illinois Filed Oct. 22, 1957, Ser.No. 691,721

3 Claims. (Cl. l1- 43) Our invention relates broadly to electricalcircuit elements of the type commonly called printed circuits, and isparticularly addressed to the fabrication by photographic .techniques ofsuch circuit elements on cylindrical forms.

One of the most popular and successful methods of making printedcircuits is the photo-etch process, in which the desired circuitconformation is defined on a thin metallic surface in the form of aphotographic image, the parts of the metal surface which are not desiredin the finished product being then etched away. The etching bath doesnot attack the parts of the metal surface which are covered by thephotographic image. After the etching, the imaged surface is Washed awaywith a suitable solvent.

This technique lends itself to low-cost mass production in applicationswhere the nished product is in the form of a flat or substantially flatsurface, since the photographic image can readily be formed on such asurface by contact printing from a negative photographic film. When,however, the finished product is in the form of a cylinder, with thedesired metal circuitry extending entirely around its outer surface, theproblem of low-cost mass production with good quality control hasheretofore defied solution.

To fashion printed circuits on cylindrical forms, it has been necessaryto wrap the negative photographic film around the cylinder and toregister the overlapping edges of the film with extremely great care.Obviously, any inaccuracy in such -registration will produce a productwhich may be worthless and which will in any event be inferior.Moreover, the carrying out of such a wrapping operation and achievementof the necessary accurate edge registration has necessarily been a handoperation, involving high labor costs and slow production.

The present invention contemplates the formation of a photographic imageof the desired printed circuit on a cylindrical form by the use of atransparent cylindrical negative which overlies the form. The entireouter surface of the negative is then illuminated simultaneously, and aphotographic image of the desired printed circuit is thus generatedsimultaneously on the cylindrical surface of the form.

As persons familiar with the art will realize, there are various kindsof circuit elements which can conveniently be fabricated in cylindricalform. Of these, perhaps the most widely used and important areinductance coils. We shall, therefore, describe our invention in thisspecification in connection with the fabrication of inductance coils andimage negatives therefor, it being understood, hoW- ever, that thevarious methods of the invention can if desired be employed in themanufacture of other types of electrical circuit elements.

In achieving our method of printing coils on a cylindrical form from apermanent cylindrical negative, we

have developed a novel method for producing a suitablecylindricalnegative image, and our invention extends to and includes suchcylindrical negative-image carrier and the process by which it isformed.

ICC

Accordingly, an important object of our invention is to provide a methodfor photographically fabricating inductance coils on cylindrical formsby photosensitizing the coil-form surface and then simultaneouslyexposing its entire operative surface to light through a cylindricalcarrier bearing an appropriate negative image.

Another object of our invention is to provide a transparent cylindricalcarrier bearing a negative image, by means of which cylindrical coilscan be photographically fabricated, such image carrier being readilyadapted for use in high-speed, automatic manufacturing processes.

Still another object of the invention is to provide a novel method bywhich such a transparent negative-image carrier can be made and anappropriate negative image placed thereon.

Other objects and advantages of the invention will appear from thefollowing description of typical applications of our methods and atypical embodiment of our cylindrical image carrier.

In connection with our description, we shall refer to the appendeddrawing, wherein various stages in the fabrication of our image carrierare illustrated. Figure l shows in axial section our image carrier in apreliminary stage of fabrication prior to formation thereon of thenegative image. Fig. 2 shows our image carrier after completion of thenegative image but prior to elimination therefrom of the temporarymandrel. Fig. 3 shows, likewise in axial section, a completed imagecarrier in accordance with our invention.

In all three of the figures of the drawing, the thickness of certainlayers therein illustrated has been intentionally exaggerated for thesake of clarity. The actual range 0f thickness of such layers will bedescribed in the specification, it being understood that the drawingsare not in that respect drawn to scale.

We shall now describe the method of preparation of a suitablenegative-image carrier which may be employed in fabricating inductancecoils wherein the helical conductor consists of a thin copper or silverlayer on a cylindrical core of low-loss insulating material such asglass.

The negative-image carrier in accordance with our invention is preparedon the temporary mandrel 10, which is a thin-walled metal tube ofcopper, brass, or aluminum. The tube 10 is cylindrical in shape andsomewhat longer than the intended coil. Thus, for a coil which is tohave a total length of two inches, the mandrel 10 might be three incheslong. The ends of the mandrel 10 are preferably enlarged slightly byswaging or other suitable operation. The purpose of this is to give thecentral aperture of the negative-image carrier funnel-shaped ends, tofacilitate insertion and removal of coil forms to be insertedtherethrough. The outer diameter of the tube 10 should be a fewthousandths of an inch larger than the outer diameter of the intendedcoil forms when they are ready for photographic processingthat is, whenthey have been coated with a thin layer of metal and an overlying layerof light-sensitive material.

The outer surface of tube 10 is polished and then coated with a layer 11of clear epoxy plastic having a thickness of about .001. It is usuallybest to apply this epoxy coating in from two to four separate coats,allowing each coat to dry beforethe next is added.

The epoxy coating 11 on the mandrel 10, which is shown on the drawingwith greatly exaggerated thickness, is then prepared for metallizing byhaving its surface very slightly roughened. This must be done with care,since the coating thickness should not be appreciably changed thereby. Arecommended procedure for this step of the process consists in rotatingthe mandrel 10 in a lathe or drill press and brushing the outer surfaceof the plastic layer 11 with a gentle abrasive.

After the gloss has been removed from coating 11 in this manner, thesurface of coating 11 should be washed with alcohol and wiped dry toremove all loose dust or other particles. The outer surface of the layer11 should be metallized by any of the standard methods commerciallyavailable, to deposit on the surface of plastic layer 11 a uniform metallayer, preferably copper, having a thickness of from .0005 to .001. Thismetal layer is shown in the drawings in greatly exaggerated thicknessand is marked with the reference numeral 12 on Fig. 1.

After the metallizing step has been completed, the surface of metalcoating 12 should be polished to insure freedom from foreign matter.This step should be done carefully to avoid damage to the thin andfragile metallic layer.

The mandrel 10, with its overlying layers 11 and 11'., should now bedipped in a bath of a suitable photo-resist compound of the type whichis normally soluble in some particular organic solvent but which isrendered insoluble in such solvent by exposure to ultraviolet light. Atypical material of this type which we have found very satisfactory isthe so-called Kodak photo-resist, sold by the Eastman Kodak Company ofRochester, New York, and commonly abbreviated KPR.

The dipping step just mentioned will result in depositing over the metallayer 12 a layer of photo-resist, marked 13 on Fig. 1. This coat shouldbe dried thoroughly.

Once the photo-resist coating has been applied, care should be taken toavoid exposure of the coated mandrel to any source of ultraviolet light,such as sunlight or untinted uorescent lamps. The Kodak photo-resistcom'- pound above mentioned can be handled in low-level incandescentlight without being substantially affected, and the same, we believe, istrue of most other types of photoresist compounds.

At this stage, the structure which is to become a negative-image carrierin accordance with our invention is at the stage of processing at whichit is ready to receive a photographic image. For this purpose, theassembly is wrapped with a photographic film 14 which carries, in sheetform, a positive image corresponding to the conformation of the desiredcoil ultimately to be fabricated. In other words, the image on the film14 represents a flattened-out version of the desired coil, dark areas onthe film corresponding to metal portions of the coil.

When such a film as film 14 is rolled into a cylindrical form, as bywrapping around the outer surface 13 of the mandrel 10, theflattened-out image on the film assumes the cylindrical shape of thecoil which it represents. In the wrapping operation, great care shouldbe taken to achieve awless registration of the overlapping edges of thefilm, to insure that each portion of the helical coil represented on oneedge of the film exactly overlies the corresponding portion on the otheredge.

While the products of other manufacturers may be used with satisfactoryperformance, we suggest, as an illustrative film 14, Eastman Kodalithtransparent stripping lm.

Once the film 14 has been accurately positioned on the outer surface ofphoto-resist layer 13, the whole assembly should be exposed toultraviolet light for a period of time appropriate to thecharacteristics of the photo-resist composition being used. Thisexposure produces in the photoresist layer 13 a negative imagecorresponding exactly to the positive image contained on film 14.

At this stage, the latent photographic image in the photo-resist layeris developed by the appropriate procedure for the composition beingused, the film 14 being of course removed prior thereto. The developingprocess` Washes away from the metallic layer 11 the photoresistcomposition in all areas not exposed, leaving a layer of insolublephoto-resist composition on the areas which were exposed to lightpassing through the film 14.

The metal layer 12 is now ready for etching. Before carrying out thatstep, however, the developed image 4 defined by the photo-resist shouldbe carefully examined under a low-power microscope to insure that it isfree from aws. Some re-touching may be carried out at this point ifnecessary. In the course of retouching care must be taken not to scratchthrough the metal and plastic layers 11 and 12.

The assembly should at this stage be dipped in a suitable etchingsolution such as ferrie chloride. The etching process is best carriedout by alternating short periods of immersion in the etching bath withwater rinses and inspections. After the etching has progressedsufficiently to remove the portions of exposed metal not covered by thedeveloped photo-resist image, the structure should be washed well inwater and then rinsed in a suitable solvent to remove the photo-resistcomposition and dye deposited therein during developing.

At this point, the metal layer 12 is no longer a continuous coating asshown in Fig. 1 but has taken the form of a negative replica of theprinted coils ultimately to be fabricated. The metal layer as thusmodified by the etching is marked 12a on Figs. 2 and 3.

By negative replica we mean that the modified rnetallic layer 12a isrelieved in all the areas which, on the completed coil, will be occupiedby conducting material, whereas the layer 12a has its metal undisturbedin the zones which, on the completed coil, will be occupied byinsulating material.

The layer 12a, after being washed and dried, should be covered by aprotective coating 15 of epoxy plastic, in a thickness of about .001 ormore. This coating, like the base coat 11, should preferably be appliedin two or three separate coats.

As the next step in preparation of our negative-image carrier, a pair ofring-shaped spacers 16 are cemented over the ends of the assembly, thespacers being cemented to the outer epoxy coat 15.

Spacers 16 may be made of any suitable hard plastic. Their outerdiameter is chosen to fit readily within the interior of a rigidtransparent tube 17 made of Lucite, glass, or other suitable material.The outer tube 17 is cemented to the spacers 16, providing a strongprotective cover for the delicate plastic cylinder that contains theimage.

One or more air vents 16a may be provided in the spacers 16 in order topermit air interchange between the atmosphere and the annulus betweenthe epoxy layer 15 and the outer tube 17.

The thickness of the tube 17 is not exaggerated in the drawing. It willnormally be sufficiently thick to possess good mechanical rigidity.Similarly, the Wall thickness of the spacers 16, which defines theextent of the annular space between the layer 15 and the transparenttube 17, may be equal or slightly greater than the thickness of the tube17. These dimensions are not critical, in any event.

If desired, the end portions of the plastic layer 15, beyond the ends ofthe wall-image proper, may be thickened by depositing additional epoxythereon. This will strengthen the structure appreciably without reducingits effectiveness, since a high degree of transparency in the layer 15is needed only for those portions of it which overlie the image-bearingzone of layer 12a.

The final step in preparing our negative-image carrier for use consistsin immersing the whole assembly in an etching solution of ferriechloride or other suitable bath, for the purpose of dissolvingcompletely away the brass mandrel 10. This operation usually requiresimmersion for several hours.

` After the brass core has completely dissolved from within the plastictube defined by layers 12 and 15, the assembly is carefully rinsed andallowed to dry. The carrier may now be used without further processing.The ends of the centralv bore will be slightly flared to permit easyinsertion of the coil forms, since the brass mandrel 10 was so formed.

If the image carrier is to be subjected to relatively heavy use, it maybe desirable to extend the cylindrical assembly for some distance beyondthe ends of the photographic image and provide a larger central bore insuch extended zones, permitting the insertion therein of a permanentfunnel guide made of metal or other rigid material. If this is to bedone, the brass mandrel will in the first instance be made substantiallylonger than is shown in the drawing, the extended ends being larger indiameter so as to produce a larger bore diameter in the completed imagecarrier. This and other modifications of the basic method and structureof our invention will suggest themselves to skilled readers from theforegoing description.

In making cylindrical printed coils with the aid of our image carrier,the coil forms, made of glass or other lowloss insulating material, aremetallized and then coated with photo-resist compound. The outerdiameter of the forms after being coated must be slightly less than theinner diameter of the central bore in our negative-image carrier.

A coil form is then inserted in the image carrier, being positionedtherein so as to place the image 12a over the desired portion of theform. The image carrier, containing the sensitized form, is then exposedfor the appropriate period to the type of radiation for which thephotoresist compound is designed, normally ultraviolet light. Theradiation impinges simultaneously on the image carrier and the form fromall radical directions, so that the entire image is formedsimultaneously on the coil form.

After exposure, the coil form is removed and subjected to the usualtreatment to develop the photographic image y thereon, after which itmay be etched.

Our cylindrical negative-image carrier is particularly well adapted tohigh-speed mass production of printed coils and other circuit elements.When our image carrier is used, the forms may be fed through the carrierin a continuous process, each coil form remaining within the carrierlong enough to be imaged and then being pushed on to the next step ofthe process bythe following coil form. Alternatively, long rods of thecoil-form material may be coated with photo-resist compound and fedthrough the image carrier in progressive steps, so that a large numberof distinct coil images are formed on a single rod of core material, therod being cut into individual coil lengths at a later stage of theprocess.

Whatever type of manufacturing process is used, our integral cylindricalnegative-image carriers aiord greatly improved quality control landreduced manufacturing costs, since the need for hand-wrapping of sheetnegatives around the cylindrical coil forms is entirely eliminated.

The detailed description herein of a typical embodiment of ourcylindrical image carrier and of the method of making it is intended tobe illustrative rather than limiting. Many variations in and departuresfrom the specic apparatus and method steps described may be made byskilled readers without departing from the scope and spirit of ourinvention.

We claim:

1. As an article of manufacture, a cylindrical image carrier comprisinga thin laminated cylinder of transparent plastic, saidimage beingdefined by a thin metal layer sandwiched between said laminations, andan outer protective holder for said image-carrying cylinder, said outerholder being transparent in the portion thereof overlying said image andbeing apertured to define with said image-carrying cylinder a continuouspassage coaxial with said cylinder.

2. As an article of manufacture, a cylindrical image carrier for use infabricating printed-circuit elements on cylindrical forms, comprising athin integral cylinder made of transparent plastic, a thin metal layerdefining said image being embedded within said plastic, and an outerprotective holder for said cylinder, said holder being substantiallyrigid and being transparent in the portion thereof overlying said metallayer, said holder being apertured to define with said cylinder acontinuous bore dimensioned to receive slidably said cylindrical forms.

3. The method of making a transparent photographic image carrier for usein fashioning cylindrical printedcircuit elements such as inductancecoils, comprising the steps of depositing a thin coating of transparentplastic on a cylindrical mandrel, metalizing the surface of said plasticcoating, depositing a layer of photo-resist compound over said metalizedsurface, exposing said layer to light through an image-carrying film toproduce therein a latent image representing said printed-circuitelement, developing the latent image in said photo-resist layer, etchingsaid metalized surface to remove the same in all areas thereof notcovered by said developed image, coating said etched metalized surfacewith a thin layer of transparent plastic, thus sandwiching said etchedsurface between said rst and second layers of plastic, mounting saidcoated mandrel within a rigid transparent protective cover whollyenclosing said etched metalized surface and circumferentially abuttingsaid second plastic layer in two spaced zones, sealing said covercircumferentially to said second plastic layer in both of said zones,and dissolving away said mandrel.

References Cited in the tile of this patent UNITED STATES PATENTS393,867 Tompsett Dec. 4, 1888 2,491,386 Miller et al Dec. 13, 19492,499,577 Fine et al Mar. 7, 1950 2,533,454 Gresham Dec. 12, 19502,544,237 Reese Mar. 6, 1951Y 2,692,190 Pritikin Oct. 19, 1954 2,711,983Hoyt June 28, 1955

3. THE METHOD OF MAKING A TRANSPARENT PHOTOGRAPHIC IMAGE CARRIER FOR USEIN FASHIONING CYLINDRICAL PRINTEDCIRCUIT ELEMENTS SUCH AS INDUCTANCECOILS, COMPRISING THE STEPS OF DEPOSITING A THIN COATING OF TRANSPARENTPLASTIC ON A CYLINDRICAL MANDREL, METALIZING THE SURFACE OF SAID PLASTICCOATING, DEPOSITING A LAYER OF PHOTO-RESIST COMPOUND OVER SAID METALIZEDSURFACE, EXPOSING SAID LAYER TO LIGHT THROUGH AN IMAGE-CARRYING FILM TOPRODUCE THEREIN A LATENT IMAGE REPRESENTING SAID PRINTED-CIRCUITELEMENT, DEVELOPING THE LATENT IMAGE IN SAID PHOTO-RESIST LAYER, ETCHINGSAID METALIZED SURFACE TO REMOVE THE SAME IN ALL AREAS THEREOF NOTCOVERED BY SAID DEVELOPED IMAGE, COATING SAID ETCHED METALIZED SURFACEWITH A THIN LAYER OF TRANSPARENT PLASTIC, THUS SANDWICHING SAID ETCHEDSURFACE BETWEEN SAID FIRST AND SECOND LAYERS OF PLASTIC, MOUNTING SAIDCOATED MANDREL WITHIN A RIGID TRANSPARENT PROTECTIVE COVER WHOLLYENCLOSING SAID ETCHED METALIZED SURFACE AND CIRCUMFERENTIALLY ABUTTINGSAID SECOND PLASTIC LAYER IN TWO SPACED ZONES, SEALING SAID COVERCIRCUMFERENTIALLY TO SAID SECOND PLASTIC LAYER IN BOTH OF SAID ZONES,AND DISSOLVING AWAY SAID MANDREL.